How do injectors know which plane of the face they are injecting?

I understand that vascular occlusion is a risk of filler, particularly in "danger zones" like the nose. But people can also have filler placed too shallowly, with bad results. How do surgeons and NPS know they are injecting into the appropriate plane of the face? For example, how can they tell they are in the subcutaneous fat layer and not the dermis? Or the SMAS layer and not the subQ layer? I'd like to get filler but I have severe anxiety around the risks. Thanks!